Mesons

Mesons are intermediate mass particles which are made up of a quark-antiquark
pair. Three quark combinations are called baryons.
Mesons are bosons,
while the baryons are fermions.
Recent experimental evidence shows the existence of five-quark
combinations which are being called pentaquarks.

Meson
diagram

Table
of mesons

Index

Particle
concepts

References
Serway
Ch.
47

Giancoli
Ch.
32

Griffiths

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pion

Particle

Symbol

Anti-
particle

Makeup

Rest mass
MeV/c^2

S

C

B

Lifetime

Decay Modes

Pion

ud

139.6

0

0

0

2.60x10^-8

Pion

Self

135.0

0

0

0

0.83x10^-16

The neutral pion decays to an electron, positron, and gamma ray by the
electromagnetic interaction on a time scale of about 10-16
seconds. The positive and negative pions have longer lifetimes of about
2.6 x 10-8 s.

The negative pion decays into a muon and a muon antineutrino as
illustrated below. This decay is puzzling upon first examination because
the decay into an electron plus an electron antineutrino yields much more
energy. Usually the pathway with the greatest energy yield is the
preferred pathway. This suggests that some symmetry is acting to inhibit
the electron decay pathway.

The symmetry which suppresses the electron pathway is that of angular
momentum, as described by Griffiths. Since the negative pion has spin
zero, the electron and antineutrino must be emitted with opposite spins to
preserve net zero spin. But the antineutrino is always right-handed,
so this implies that the electron must be emitted with spin in the
direction of its linear momentum (i.e., also right-handed). But if the
electron were massless, it would (like the neutrino) only exist as a
left-handed particle, and the electron pathway would be completely
prohibited. So the suppression of the electron pathway is attributed to
the fact that the electron's small mass greatly favors the left-handed
symmetry, thus inhibiting the decay. Weak interaction theory predicts that
the fraction of muons decaying into electrons should be 1.28 x
10-4 and the measured branching ratio is 1.23 +/- 0.02 x
10-4.

The pion, being the lightest meson, can be used to predict the maximum
range of the strong
interaction. The strong interaction properties of the three pions are
identical. The connection between pions and the strong force was proposed
by Hideki
Yukawa. Yukawa worked out a potential for the force and predicted its
mass based on the uncertainty
principle from measurements of the apparent range
of the strong force in nuclei.

Being composed of an up and an antidown quark,
the positive pion would be expected to have a mass about 2/3 that of a
proton, yet it's mass is only about 1/6 of that of the proton! This is an
example of how hadron
masses depend upon the dynamics inside the particle, and not just upon
the quarks contained.

The pion is a meson.
The
is considered to be made up of an up and an anti-down quark.
The neutral pion is considered to be a combination

Pions interact with nuclei and transform a neutron to a proton or vice
versa:

The pions p+ and p- have spin zero and negative intrinsic
parity (Rohlf
Sec 17-2).

Energetics
of charged pion decay

Meson
diagram

Table
of mesons

Index

Particle
concepts

References
Griffiths
Sec
10.4

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The psi/J Particle

Particle

Symbol

Anti-
particle

Makeup

Rest mass
MeV/c^2

S

C

B

Lifetime

Decay Modes

J/Psi

Self

cc

3096.9

0

0

0

0.8x10^-20

...

The psi/J particle is a meson
which was discovered in 1974 by experimenters at Stanford
(Richter) and Brookhaven
National Laboratory (Ting). Slightly more than three times as massive as
the proton,
this particle decayed slowly and didn't fit into the framework of the up,
down, and strange quarks.
It is considered to be a charm-anticharm
quark pair and was the first firm experimental evidence for the fourth
quark. Richter and Ting shared the 1976 Nobel Prize for their discovery.

Meson
diagram

Table
of mesons

Index

Particle
concepts

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The Upsilon Particle

Particle

Symbol

Anti-
particle

Makeup

Rest mass
MeV/c^2

S

C

B

Lifetime

Decay Modes

Upsilon

Self

bb

9460.4

0

0

0

1.3x10^-20

...

The upsilon particle is a meson
which was discovered at Fermilab
in 1977. It appeared as another long-lived particle which didn't fit into
the framework of the first four quarks,
the up,down, strange, and charm quarks. It is taken as a botton-antibottom
quark pair and was the first experimental evidence of the fifth quark.

Meson
diagram

Table
of mesons

Index

Particle
concepts

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Hadrons

Particles that interact by the strong
interaction are called hadrons. This general classification includes
mesons
and baryons
but specifically excludes leptons,
which do not interact by the strong force. The weak
interaction acts on both hadrons and leptons.

Hadrons are viewed as being composed of quarks,
either as quark-antiquark pairs (mesons) or as three quarks (baryons).
There is much more to the picture than this, however, because the
constituent quarks are surrounded by a cloud of gluons,
the exchange particles for the color
force.

Recent experimental evidence shows the existence of five-quark
combinations which are being called pentaquarks.

Index

Particle
concepts

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Baryons

Baryons are massive particles which are made up of three quarks
in the standard model. This class of particles includes the proton
and neutron.
Other baryons are the lambda, sigma, xi, and omega particles. Baryons are
distinct from mesons
in that mesons are composed of only two quarks. Baryons and mesons are
included in the overall class known as hadrons,
the particles which interact by the strong
force. Baryons are fermions,
while the mesons are bosons.
Besides charge and spin (1/2 for the baryons), two other quantum numbers
are assigned to these particles: baryon number (B=1) and strangeness (S),
which in the chart can be seen to be equal to -1 times the number of
strange quarks included.

The conservation
of baryon number is an important rule for interactions and decays of
baryons. No known interactions violate conservation of baryon number.

Recent experimental evidence shows the existence of five-quark
combinations which are being called pentaquarks.
The pentaquark would be included in the classification of baryons, albeit
an "exotic" one. The pentaquark is composed of four quarks and an
antiquark, like a combination of an ordinary baryon plus a meson.

Table
of Baryons

Baryon
diagram

Index

Particle
concepts

Reference
Serway
Ch.
47

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Hideki Yukawa and the Pion

Once quantum electrodyamics had produced the picture of the electromagnetic
force as a process of exchanging photons, the question of whether or
not the other forces were also exchange
forces was a natural one. In 1935, Hideki Yukawa reasoned that the
electromagnetic force was infinite in range because the exchange particle
was massless. He proposed that the short range strong
force came about from the exchange of a massive particle which he
called a meson. By observing that the effective
range of the nuclear force was on the order of a fermi, a mass for the
exchange particle could be predicted using the uncertainty principle. The
predicted particle mass was about 100 MeV. It did not receive immediate
attention since no one knew of a particle which fit that description.

In 1937 a particle of mass close to Yukawa's prediction was discovered
in cosmic rays by Anderson & Neddermeyer and by Street & Stevenson
in independent experiments. This particle, the muon,
turned out not to interact by the strong interaction. Hans Bethe and
Robert Marshak predicted that the muon could be a decay product of the
particle sought. In 1947, Lattes, Muirhead, Occhialini and Powell
conducted a high altitude experiment, flying photographic emulsions at
3000 meters. These emulsions revealed the pion,
which met all the requirements of the Yukawa particle.

We now know that the pion is a meson,
a composite particle, and the current view is that the strong interaction
is an interaction between quarks, but the Yukawa theory stimulated a major
advance in the understanding of the strong interaction.

Index

References
Rohlf,
Ch
17.

Lattes, et
al.

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